Phase diagram of vortices in the quasi-two-dimensional organic superconductor α−(BEDT−TTF)2NH4Hg(SCN)4: A system of pancake vortices with out-of-plane coupling dominated by the electromagnetic energy

Abstract

Anisotropy and field-temperature (H−T) phase diagram of the quasi-two-dimensional superconductor, α−(BEDT−TTF)2NH4Hg(SCN)4, were investigated through the specific heat, ac complex susceptibility χ=χ′−iχ″, and resistivity measurements under zero field and dc fields normal to the conducting plane. The anisotropy parameter γ was determined to be as large as ∼103. In the vortex state of this highly anisotropic superconductor, it was found that the onset of the in-plane and out-of-plane ac susceptibility forms a characteristic line well below the Hc2 line determined by the specific-heat measurements. The absence of the frequency dependence in the out-of-plane susceptibility suggests that this is a melting line of vortices, rather than a crossover of vortex dynamics. The out-of-plane resistivity measurements show that the interlayer coherence between pancake vortices is established well above the melting line. Considering the anisotropy parameter obtained in the present study, the pancake vortices in this superconductor is viewed as an assembly coupled mainly through the electromagnetic energy in the out-of-plane direction. According to this feature, the two characteristic lines in the H−T diagram were qualitatively explained by the recent melting and decoupling theories taking account of the electromagnetic coupling.

Source-reported materials — not catalogue approval

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